\(^{14}\)N Nuclear Magnetic Resonance Studies of Small Molecules and Biological Samples

Abstract

Nitrogen is one of the most important atoms in all of chemistry and biology and nuclear magnetic resonance (NMR) spectroscopy is one of the most powerful tools available to chemists and biologists today. However, the major isotope of nitrogen (\(^{14}\)N,99.7% abundance) has not been studied in much detail. The major reason for this lack of research is due to the fact that \(^{14}\)N, as a spin-1 nucleus, has a very large quadrupole moment. As a result, molecular motion can very efficiently and quickly relax an excited spin. This leads to extremely wide peaks, often wider than 100 Hz at half height, which are invisible under most typical concentrations and experiment times. However, through an extensive assay of nitrogen-containing molecules, a handful of groups were shown to exhibit sharp peaks such as (nearly) symmetrically substituted ammonium ions, azides, isocyanides, and some nitriles and nitro-containing compounds. As a result, it was found that \(^{14}\)N NMR could be a highly selective spectroscopy method. To test this selectivity, several biological materials such as scallops and veal liver were assayed for choline, an important biomarker, using 1D direct-detection and newly developed 2D \(^{1}\)H-\(^{14}\)N HSQC inverse-detection experiments. These spectra have exhibited five peaks or fewer in both experiments, as compared to the dozens that would appear in the proton or \(^{13}\)C spectra. This simplifies identification and quantification of molecules like choline in very complex mixtures. It is envisioned that these experiments could be used in a wide variety of niche applications such as the quantification of choline in cells for the detection of cancer. Future work will be focused on the development of new experiments and the fulfillment of the promising applications \(^{14}\)N NMR can bring into the realms of academia, medicine, and industry.